Concentrated solar power (CSP) involves the use of lenses, mirrors or other optical devices to focus solar radiation from a large incident area onto a small area. The energy from the solar radiation is then used to generate electrical power. Concentrated solar power has the potential to become an important energy source in the future.
There have been many proposals for concentrated solar power technology. The technology believed to have the most potential for providing high efficiency power generation is the concentrated solar thermal power system. This technology involves the use of a solar receiver steam generator, mounted atop a tower, onto which solar radiation is reflected by an array of tracking reflectors, such as heliostats forming a heliostat field around the tower. The reflected solar radiation directly heats water circulating through the solar receiver steam generator. This generates superheated steam which is used to drive a steam turbine generator set, and thereby generate electrical power, the Rankine cycle.
Concentrated solar thermal power systems can include energy storage capability so that they can continue to generate electrical power when the solar radiation reflected onto the solar receiver steam generator is not sufficient to generate steam at the desired pressure and temperature to drive the steam turbine generator set. The energy storage capability can be provided by a thermal energy storage arrangement which uses a high specific heat capacity thermal energy storage fluid, such as molten salt or a mixture of different molten salts. Thermal energy is stored during a charging cycle by heating the molten salt and the thermal energy is subsequently recovered during a discharging cycle to heat water, and thereby generate steam for the steam turbine generator set.
During a first operating mode of the solar thermal power system, a proportion of superheated steam generated by the solar receiver steam generator is supplied directly to the high pressure turbine inlet of the steam turbine generator set for electrical power generation. The remaining superheated steam is supplied to the thermal energy storage arrangement to support the charging cycle in which a heat exchanger is used to extract thermal energy from the superheated steam and transfer it to the molten salt. The heated molten salt is stored in an insulated storage container. During a second operating mode of the solar thermal power system when superheated steam is not supplied by the solar receiver steam generator to the steam turbine generator set or to the thermal energy storage arrangement, previously stored thermal energy is recovered from the hot molten salt during a discharging cycle by a heat exchanger and the recovered thermal energy is used to heat water, and thereby generate superheated steam. This superheated steam is supplied to the steam turbine generator set, again via the high pressure turbine inlet, to generate electrical power.
In order for solar thermal power systems of this type to operate at maximum efficiency and maximum power output, the steam generated by the thermal energy storage arrangement during the second operating mode should ideally have the same temperature and pressure as the steam generated by the solar receiver steam generator during the first operating mode. However, because of the manner in which the molten salt is heated during the first operating mode by extracting thermal energy from the superheated steam in a heat exchanger, the maximum temperature that can be attained by the molten salt is lower than the maximum temperature of the steam from which the thermal energy is extracted. This results in energy loss (also known as a ‘pinch point loss’) and occurs because as the superheated steam cools in the heat exchanger during the charging cycle, it changes state to condensed water. During this change of state when latent heat is released, the temperature of the steam does not decrease but the temperature of the molten salt increases monotonically. Due to this mismatch of thermal behaviour between the two fluids in the heat exchanger, the highest temperature that the molten salt can attain is lower than the temperature of the incoming superheated steam, leading to reduced energy.
Consequently, when thermal energy is subsequently recovered from the molten salt during the second operating mode of the solar thermal power system to generate steam for the steam turbine generator set, the generated steam attains a lower pressure (possibly greater than 20% lower) and a lower temperature (possibly greater than 30° C. to 100° C. or more lower depending on the thermal energy storage arrangement that is used) than the superheated steam that was used to heat the molten salt and drive the steam turbine generator set during the first operating mode. Because steam at sub-optimal pressure and temperature is supplied to the steam turbine generator set via the high pressure turbine inlet during the second operating mode, the steam turbine generator set operates in a part-load condition which reduces power output of the solar thermal power system. It is not uncommon for the power output to be more than 20% lower during the second operating mode than during the first operating mode (when a proportion of the superheated steam from the solar receiver steam generator is supplied directly to the high pressure turbine inlet of the steam turbine generator set).
It would, therefore, be desirable to provide a solar thermal power system having improved operational efficiency and power output, for example, during a second operating mode when steam is generated by recovering stored thermal energy from a thermal energy storage fluid of a thermal energy storage arrangement.